The present invention relates to printed circuit boards and to methods of manufacturing printed circuit boards. More specifically, the invention relates to printed circuit boards including via holes and formed of a photosensitive resin. The invention also relates to photomasks for use in such manufacture.
As is known, printed circuit boards (PCBs) are used in various kinds of electronic equipment. Additionally, printed circuit boards which use via holes in order to provide electrical connections across interim insulating layers have been used to meet high density wiring requirements in many of today""s equipment designs.
FIG. 8 is a sectional view of a conventional PCB 1 using a via hole, the PCB including a base substrate 2 and a pair of built-up portions 4 formed of insulating resin layers 3, and located on both sides of the interim base substrate 2. In each of these built-up portions 4, via holes 5 and conductive circuit patterns 6 are formed. The base substrate 2 is formed of, for example, a multi-layered composite, which includes, for example, a glass epoxy substrate, or the like, with spaced internal conductive layers.
Each of the via holes 5 shown in FIG. 8 is formed in order to provide electrical connection across a thickness of the insulating resin layers 3. Heretofore, this via hole 5 was typically formed by a boring method such as mechanical drilling, laser drilling, or a photolithography method (hereinafter, referred to as a xe2x80x9cphoto via methodxe2x80x9d). Forming the resin layer 3 of portion 4 may be accomplished by resin coating, laminating a film-shaped resin, and the like. Of these methods, the photo via method, which uses photosensitive resins and photolithographic processing, is considered particularly suitable to meet the high densification and high integration requirements for such boards.
FIG. 9 shows a manufacturing method for a PCB, in which the via holes are formed by a conventional photo via method. As shown in FIG. 9, an insulating photosensitive resin is provided (coated on a base substrate having a fine circuit formed on its surface) in Step 901. Next, in Step 902, imaging exposure is performed on the photosensitive resin with ultraviolet rays through a photomask. Then, in Step 903, the unexposed and uncured photosensitive resin is developed and removed by use of known liquid developer, to form a via hole. Thereafter, in Step 904, the resin layer is subjected to heat treatment to stabilize various characteristics thereof. A surface roughening treatment of the layer next occurs, followed by plating, e.g., using a method such as non-electrolytic plating. Circuit formation is performed in Step 907, thus forming a fine circuit. These steps can be repeated any number of times depending on the number of layers desired, to thus produce a built-up multi-layered PCB.
In recent years, via holes (i.e., 5 in FIG. 8) formed in the above-described process have been manufactured as small in diameter as defined by the resolution limit of the insulating photosensitive resin used for the layers. Occasionally, there occurs unevenness in the thickness of the photosensitive resin layer. If this occurs in an area where an underlying conductive circuit pattern is present, the photosensitive resin forming the insulating layer 3 may not be fully developed and removed, possibly resulting in incomplete via hole formation, and thus a defective final product. Understandably, this can prove extremely costly when producing PCBs having many individual layers. The defect may be in the form of a failed connection, e.g., to the conductive circuit pattern formed on the layer""s upper surface. Accordingly, to meet the high density wiring requirements of today""s PCBs, uneven layer thicknesses must be minimized; that is, the insulating layers must be as flat (planar) as possible. It has been determined that a thickness variation of 5 micrometers or less, (preferably 3 micrometers or less) is acceptable for many of today""s designs. Heretofore, the film unevenness has reached about 17 micrometers in some cases, depending on the shape and the thickness of the conductive circuit pattern and the properties of the photosensitive resin. To reduce such unevenness, polishing the surface of the photosensitive resin layer before the aforementioned developing step has occurred. However, this polishing step has resulted in scratch formation in the surface of the photosensitive resin layer, which can adversely affect the subsequent developing step.
It is believed, therefore, that a new method of making a PCB which overcomes the problems described above would constitute an advancement in the art. The resulting board and the mask used in such a process would also represent art advancements.
A primary object of the present invention is to provide an improved PCB construction and method for making same.
Another object is to provide a method which assures greater process yields, without causing incomplete via holes during the formation of via holes, and further without defects such as a circuit short or a circuit disconnect during formation of a subsequent conductive circuit pattern.
Further, another object of the present invention is to provide a new and improved photomask adaptable for use in such a method.
Specifically, according to one aspect of the present invention, provided is a PCB comprising a base substrate including a conductive circuit pattern on a surface thereof and at least one resin layer provided on the base substrate, wherein the resin layer is formed of a photosensitive resin. The printed circuit board is formed by exposing the photosensitive resin layer on the conductive circuit pattern with an amount of exposure dependent on the area of the associated conductive circuit pattern.
According to another aspect of the invention, there is provided a method of manufacturing a PCB which comprises the steps of coating a photosensitive resin on a base substrate including a conductive circuit pattern on a surface thereof. Light is exposed onto a position of the conductive circuit pattern with an exposure amount in accordance with an area of the conductive circuit pattern. The exposed photosensitive resin is developed to remove a layer of the photosensitive resin on the conductive circuit pattern in accordance with the area of the conductive circuit pattern, thus setting a film thickness difference of the layer of the photosensitive resin from the surface of the base substrate to 5 micrometers or less.
According to yet another aspect of the invention, there is provided a photomask for exposing a photosensitive resin layer, which comprises a light shielding portion corresponding to a portion of the photosensitive resin layer below which the conductive circuit pattern is not formed, an exposure quantity adjusting portion formed of a fine pattern in accordance with an area of the conductive circuit pattern in a position of the conductive circuit pattern, and a light shielding portion for forming a via hole.